Every now and then I look at things like the proxycache overlay with its
internal database, and wonder if there would ever be a need to put an
overlay on that database. I suppose one way to handle that would be to
add explicit proxycache directives for configuring such a feature.
Another thought that came to mind was being able to declare overlays
inside a "backend" section, such that these overlays would automatically
be instantiated whenever a particular type of backend is used. Comments?